1. Field of the Invention
The invention relates generally to non-invasive methods and systems for probing the earth, and more specifically to radars that can image and detect objects and other anomalies in the ground.
2. Description of the Prior Art
Many valuable and/or dangerous objects are buried in the ground, and digging them up to see what is there is often not possible or practical. Ground-penetrating radars have been developed as a way to xe2x80x9cseexe2x80x9d what is underground. But conventional methods and equipment have only provided crude hints of things very near the surface.
Michael D. Bashforth, et al., describe a wide band stepped frequency ground penetrating radar in U.S. Pat. No. 5,499,029, issued Mar. 12, 1996. Such relates to attempts to increase the average signal power and to preserving phase information so digital signal processing can extract more information about objects in the soil. The radar transmitter steps in frequency from 100 MHz to 1,000 MHz, and data is taken at 2.0 MHz step intervals. Both in-phase and quadrature data are collected for over 900 samples. The received signals are combined with samples from the transmitter to detect any phase shifting that may have been caused by objects in the ground, e.g., landmines and waste containers.
An earlier technology is described in U.S. Pat. No. 5,325,095, issued Jun. 28, 1994, to Kenneth G. Vadnais, et al. Such discloses a prior art stepped-frequency ground penetrating radar with a less-capable phase locked loop device.
The present inventors, Larry Stolarczyk and Gerald Stolarczyk, describe the measuring of the thickness of ground deposit layers with a microstrip antenna, in U.S. Pat. No. 5,072,172, issued Dec. 10, 1991. Interpolation tables are used to lookup the layer thickness values corresponding to antenna conductance and resonance measurements. Such resonant microstrip patch antenna (RMPA) and their resulting measurements are used to guide coal-seam drum-cutter equipment for more efficient mining of natural deposit ores. The RMPA driving-point impedance (S11) changes significantly when a solid, gas, or liquid layer thickness overlying the RMPA varies.
The RMPA can be swept above a soil surface to find buried landmines, utilities, and other shallow-buried objects. These objects don""t necessarily need to be made of metal to be found. What is needed is that the dielectric constants of the objects and the medias they are buried in must differ, e.g., for contrast.
Calibrating the RMPA sensor establishes the RMPA driving-point impedance relationship to a layer-measurement value. In prior art mining applications, layers of various thickness needed to be cut with a mining machine so the RMPA impedance at each layer thickness can be recorded. But such prior art calibration procedures proved to be difficult in practice. A better approach is needed that has an independent means of measuring layer thickness that can be run concurrent to any driving-point impedance measurements.
U.S. Pat. No. 5,769,503, issued Jun. 23 1998 to Stolarczyk, et al., describes mounting such RMPA on a rotating drum or arm of a coal, trona, or potash mining machine. A ground-penetrating-radar transmitting antenna and a receiving antenna can be mounted on a cutting drum to detect deeply buried objects and anomalous geology just ahead of the mining. A radar frequency downconverter is used so low-cost yet-accurate measurement electronics can be built. A first phase-locked loop (PLL) is operated at the resonant frequency of the patch antenna or at each sequentially stepped radar frequency. A second PLL is offset from the first PLL by an intermediate frequency (IF) and is called a tracking PLL. The measurement speed can be delayed by the sequential way in which the PLL""s lock on to signals, so a solution to that delay is described.
The calibration curves represent an analytical function that has been reconstructed from a set of discrete I and Q data points measured at each height (H). The discrete sensor height calibration data can be used to construct two different polynomials with the independent variable being the physical layer thickness or height (H). The physical height (H) is independently measured with acoustic height measurement electronics during the calibration process or by other means, such as an inclinometer on the boom of a mining machine. The two calibration polynomials are,
I(H)=Re H=bnHn+bn-1Hn-1+. . . +b1H+bOxe2x80x83xe2x80x83(1A)
and
Q(H)=Im H=anHn+an-1Hn-1+. . . +a1H+aOxe2x80x83xe2x80x83(1B)
U.S. Pat. No. 5,325,095 describes a modulator that sequentially creates in-phase (I) and quadrature phase (Q) shifts in a frequency source signal. The frequency source signal is sequentially shifted by 0xc2x0 or 180xc2x0(in-phase), then by 90xc2x0or 270xc2x0(quadrature) in passing through the phase modulator to the radar transmit antenna. The electronic circuits employ isolators. Isolators and quadrature modulator transmitters are costly and difficult to build with wide bandwidth. The receiver section of the radar receives the reflected signals from the target and uses a single frequency conversion design to transpose the received radar signal frequency to a lower frequency range where the I and Q signal measurements are sequentially made at each frequency in the stepped-frequency radar method that has become one of the standard ground penetrating radar practices. The I and Q signals contain the antenna sensor information. As is well known in the art, the sensor information is processed in a Fourier transform to transform frequency domain information to time domain information. The time domain information is used to determine the time (tO) for the signal energy to travel to and return back to the radar. By knowing the velocity (V) in a dielectric natural media such as coal   v  =      c                  ϵ        c            
where c is the speed of light, xcex5c is the relative dielectric constant of coal (about 6). The distance to the reflective target is   d  =            c              2        ⁢                              ϵ            c                                ⁢                  t        o            .      
The relative dielectric constant must therefore be known to accurately to determine distance.
The velocity formula is made more complex whenever the natural media layer is not coal, trona, or some other high-resistivity liquid or solid. The velocity of radio waves generally depends on the frequency and resistivity of the natural medium. It is therefore preferable to simultaneously measure the in-situ dielectric constant, e.g., when using radar to measure depths. Stepped-frequency radars have separate transmitting and receiving antennas, and are circularly polarized antennae. But printed circuit antennas radiate front and back. To counter this, U.S. Pat. No. 5,325,095, teaches the placement of radar-energy absorbing material on one side of the printed circuit board to reduce the back lobe.
The antenna pattern is directed only to one side of the printed circuit antenna. Such antennas are preferably oppositely polarized so that they can be operated in continuous wave (CW) mode and in close proximity to each other. The transmitter and receiver sections operate concurrently. The radar return signals from the target will typically be repolarized opposite to the transmitted signal. The reflected wave can thus be readily measured by the receiving antenna and associated electronics. But not all the reflected signals will be oppositely polarized. An electromagnetic wave traveling in a first media and into a second media is reflected at the interface.
Electromagnetic wave reflection occurs at the interface of two different dielectric medias, and the reflection coefficient can be expressed in Equation (2) as,                               Γ          =                                                    E                s                                            E                p                                      =                                                                                ϵ                    1                                                  -                                                      ϵ                    2                                                                                                                    ϵ                    2                                                  +                                                      ϵ                    1                                                                                      ;                              σ            ωϵ                    ⁢                       less than  less than             1                                              (        2        )            
where, Es is the reflected electric field component of the electromagnetic wave, a vector; Ep is the incident electric field component of the electromagnetic wave, a vector; xcex51 is the relative dielectric constant of the first media; xcex52 is the relative dielectric constant of the second media; "sgr" is the electrical conductivity of the media; and, xcfx89=2xcfx80f and f is the frequency of the EM wave.
When the dielectric constant of a first-encountered media is greater than a second-encountered media, the reflection coefficient will be positive. The radars described in U.S. Pat. N0. 5,325,095 and 5,499,029 are not effective. There is a need to build a radar antenna that will not be limited in this way.
Whenever the energy applied to the antenna sensor for measurement purposes is changed, there is a finite ring-down time period when measurements cannot be made. After each frequency step, the measurement circuit must wait to the end of the ring down time period before the measurement can be made, and this limits the speed of measurement. When a resonant microstrip patch or radar antenna is positioned in close proximity to the surface of natural media, the driving-point impedance changes from the free-space value. At very close proximity, a severe impedance mismatch condition develops between the transmitter and the antenna sensor. In any event, some of the reflected waves returning from the antenna sensor will propagate back into the modulator and unbalance it. These imbalances can adversely affect the in-phase and quadrature signal generation process. The modulator needs to be isolated from such reflected waves.
The measurement of antenna impedances can be complicated by three-port directional couplers. The observable is the reflection port output signal eR of a three-port directional coupler that has been connected to the antenna sensor. This can be used to make measurements of natural media parameters, to detect non-metallic and metallic objects, natural media thickness, in-situ stress, and even the dielectric constant of the media. The reflection coefficient (xcex93) is defined as,                     Γ        =                                            e              R                                      e              i                                =                                                    Z                L                            -                              Z                o                                                                    Z                o                            +                              Z                o                                                                        (        3        )            
where, ei the signal applied to the directional coupler; eR is the reflection port output signal; Z0 is the characteristic impedance of the circuit driving the antenna; and ZL is the driving-point impedance of the antenna.
Assuming ei is constant, the in-phase (I) and quadrature (Q) components of the three-port directional coupler reflective port signal eR can be measured. Equation (1) can be normalized with respect to ZO as:                     Γ        =                                            e              R                                      e              i                                =                                                    (                                                      Z                    L                                                        Z                    o                                                  )                            -              1                                                      (                                                      Z                    L                                                        Z                    o                                                  )                            +              1                                                          (        4        )            
Maximum detection sensitivity occurs when ZL is near ZO, e.g., the characteristic impedance of the three-port directional coupler. The detection sensitivity is defined as the ratio of an incremental change in coupler output voltage (xcex94eR) to an incremental change in impedance (xcex94ZL). The three-port directional coupler detection sensitivity can be determined from the derivative of eR versus {fraction (ZL/ZO)} characteristic. The detection sensitivity reaches a maximum value when ZL=ZO. Here, a xcex94ZL change produces a xcex94eR change in reflection port output voltage. If ZL is either very high or very low, then there is practically no change in reflected voltage (xcex94eR) with changes in RMPA driving-point impedance.
The transmission line input impedance (Z1n) of a coaxial cable is,                               Z          in                =                              [                                                            Z                  L                                +                                                      iZ                    o                                    ⁢                                      xe2x80x83                                    ⁢                  tan                  ⁢                                      xe2x80x83                                    ⁢                  β                  ⁢                                      xe2x80x83                                    ⁢                  l                                                                              Z                  o                                +                                                      iZ                    L                                    ⁢                                      xe2x80x83                                    ⁢                  tan                  ⁢                                      xe2x80x83                                    ⁢                  β                  ⁢                                      xe2x80x83                                    ⁢                  l                                                      ]                    ⁢                      Z            o                                              (        5        )            
where, xcex2 is the phase constant of the transmission line, and l is the length of the transmission line. If the line length can be made very short, then Zin=ZL.
The transmission line transforms the impedance. For example, if the line is a quarter wavelength, one frequency is terminated by a short or low impedance value and becomes a high input impedance value Zin. Clearly, it would be preferable to have an antenna sensor input port measuring circuit that would not be bandwidth limited by an isolator and phase quadrature modulator, and highly sensitive to any RMDA driving-point impedance variation caused by operating it in close proximity to the earth. It would also be preferable not to have the problem of the reflected wave being oppositely polarized from that of the receiver antenna, thus limiting detection sensitivity.
The prior art fails to teach improvements in detection sensitivity that can be achieved when the antenna sensor is in close proximity to natural media. For example, where the measuring circuit is a coupler located in close proximity to the antenna driving-point and the driving-point impedance of the antenna sensor is equal to the characteristic impedance of the coupler.
A single radar antenna is preferred over two antennas, e.g., as were required in the description in U. S. Pat. No. 5,325,095. The relative dielectric constant of media is also preferably measured simultaneously so that the correct relative dielectric constant can be used in determining measurement distance.
What is needed is a method of optimizing the detection and measurement of the RMPA driving-point impedance and reflected signals of an antenna sensor operated in close proximity to natural media. Also needed is a measuring system that correctly maintains the amplitude and phase information in the signal reflected from the antenna sensor and that enables measurement at low frequency, e.g., for low-cost implementations. The prior art further lacks improved RMPA and radar antenna sensors calibration methods.
The mining industry could benefit from being able to detect and image objects and anomalous geology well in advance of drilling and extraction. New methods for measuring the dielectric constant of thick layers and the in-situ stress at the cutting edges of drills and mining machines are needed. The prior art has not provided a radiowave means of sending measured data from the drum or drill-rod that includes vibration, bit force, and temperature.
The military would benefit from being able to safely detect and locate the new type of anti-tank and anti-personnel landmines that use plastic housings instead of metal.
Briefly, a ground-penetrating radar embodiment of the present invention comprises a single RMPA that is driven by a three-port directional coupler. A reflected-wave output port is buffered by a wideband isolation amplifier and a reflected-wave sample is analyzed to extract measured values of the real and imaginary parts of the load impedance. Each such part will vary in a predictable way according to how deeply an object is buried in the soil. Calibration tables can be empirically derived. Reflections also occur at the interfaces of homogeneous layers of material in the soil. The reflected-wave signals are prevented from adversely affecting transmitted-signal sampling by putting another wideband isolation amplifier in front of the input port of the directional coupler. A suppressed-carrier version of the transmitted signal is mixed with the reflected-wave sample, and the carrier is removed. Several stages of filtering result in a DC output that corresponds to the values of the real and imaginary parts of the driving point impedance of the antenna sensor. The suppressed-carrier version of the transmitted signal is phase shifted 0xc2x0 or 90xc2x0 to select which part is to be measured at any one instant.
An advantage of the present invention is that a ground-penetrating radar is provided that finds objects and layer boundaries in the ground and ore bodies.
An advantage of the present invention is that a three-port directional coupler and low-cost balanced mixer can be used to transpose the antenna sensor signal to low frequency signal where accurate and low-cost synchronous detection electronics can be applied in the measurement method.
Another advantage of the present invention is that antenna impedance variation that gives rise to reflected waves in the transmit signal path causes minimal effect in the transmit circuits.
Another advantage of the present invention is that the identical antenna sensor measuring circuit can be used with RMPA and microwave horn antennas.
An advantage of the present invention is that the transmit, modulator, and balanced mixer signals are generated with reference to a common crystal oscillator to ensure phase coherence throughout the measurement process.
An advantage of the present invention is that the RMPA sensor can be calibrated to measure overlying dielectric constant and when used with a radar signal processing algorithm enables the accurate measurement of distance through natural media.
An advantage of phase-coherent generation is that an optimum instrument design is realized with respect to the sinusoidal signal embedded in electrical noise. The measurement advantage of synchronous detection are realized in the design.
An advantage of the present invention is that the measured data can be transmitted from the measurement location by radiowaves.
An advantage of the present invention is that a lower cost and higher quality measuring instrument can be provided.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.